1. Field of the Invention
The present invention relates to a light-emitting element which can convert electric energy to light to emit light, in particular, to an organic electroluminescent element and novel compounds to be used therein.
2. Description of the Related Art
An organic electroluminescent (EL) element has attracted attention since it can provide luminescence with a high luminance at a low voltage. As an important characteristic value of this organic electroluminescent element, there exists external quantum efficiency. The external quantum efficiency is calculated by the formula of “external quantum efficiency φ=a number of photons emitted from an element/a number of electrons injected into the element”.
An element having higher external quantum efficiency can be said to be advantageous in view of electric power consumption.
The external quantum efficiency of an organic electroluminescent element is determined according to the formula of “external quantum efficiency φ=internal quantum efficiency×light extraction efficiency”. In an organic EL element utilizing fluorescent luminescence from an organic compound, the limit value of internal quantum efficiency is 25%, and light extraction efficiency is about 20%, and hence the limit value of external quantum efficiency is believed to be about 5%.
As a method of improving external quantum efficiency of an element by improving the internal quantum efficiency of an organic electroluminescent element, an element (a triplet light-emitting element) using a triplet light-emitting material (phosphorescent luminescence material) has been reported (WO00/070655). In comparison with conventional elements utilizing fluorescent luminescence (singlet light-emitting element), this element can improve the external quantum efficiency, and the maximum value of the external quantum efficiency of an element using Ir(ppy)2 which is a green-color phosphorescent material has amounted to 8% (external quantum efficiency upon 100 cd/m2: 7.5%). On the other hand, in order to apply an element using a triplet light-emitting material to a full-color display or a white light-emitting element, there has been demanded an element capable of generating luminescence with a high efficiency in the other colors, particularly in the blue region.
An element has recently been reported which uses a light-emitting material capable of emitting a light of 500 nm or less in maximal luminescence and a host material having the lowest triplet energy level lower than that of the light-emitting material (JP-A-2002-100476). In comparison with conventional host materials, this element can improve the external quantum efficiency, but has been demanded to improve in view of durability.
Further, there has been proposed an organic electroluminescent element employing a compound in which a plurality of imidazole skeletons is connected through a linking group (JP-A-2005-89544), but there are problems in that a lowest triplet energy level is low and when it is employed as a material for a triplet light-emitting element (particularly as a material constituting a light-emitting layer or a layer adjacent to the light-emitting layer), a luminous efficiency for the element lowers.